Recently, music distribution using high-resolution music which is audio data having higher sound quality than a music CD (CD-DA) has been performed. For pulse code modulation (PCM) music such as 96 kHz/24 bits, there is a lossless compression technology (reversible compression technology) such as free lossless audio codec (FLAC) that is used for distribution.
Further, for ΔΣ-modulated digital signals (direct stream digital (DSD) data) which are not PCM signals but are 1-bit signals, there is a technology called direct stream transfer (DST) developed by Philips as a lossless compression technology and this technology is used to manufacture a super audio CD (SACD).
However, this technology is based on 1-bit signal processing and is not suitable for software processing in a CPU based on processing in bytes, and thus is mounted as hardware (LSI) in an SACD player and the like. Since high throughput is required for processing through software, processing cannot be performed in a general embedded CPU.
Accordingly, there is a demand for a lossless compression technology which enables processing even in a general embedded CPU while considering execution by mobile terminals when audio signals are distributed using DSD data.
The applicant has proposed a technology for compressing current data into 2 bits with reference to past data in units of 4 bits in Patent Literature 1 as a lossless audio signal compression technology using DSD data.